Fifth book of lessons for the use of schools . 83* 338 FIFTH BOOK. of paper held in this focus will take fire; for rays oflight cannot be concentrated without accumulating aproportional quantity of heat; hence concave mirrorshave obtained the name of burn-ing mirrors. If a burning taperbe placed in the focus, the raywhich falls in the direction ofthe axis of the mirror will bereflected back in the same line;but two other rays, drawn fromthe focus, and falling on themirror at B and F, will be reflected to A and E.—Therefore the rays which proceed from a light placedin the focus of a concave mir
Fifth book of lessons for the use of schools . 83* 338 FIFTH BOOK. of paper held in this focus will take fire; for rays oflight cannot be concentrated without accumulating aproportional quantity of heat; hence concave mirrorshave obtained the name of burn-ing mirrors. If a burning taperbe placed in the focus, the raywhich falls in the direction ofthe axis of the mirror will bereflected back in the same line;but two other rays, drawn fromthe focus, and falling on themirror at B and F, will be reflected to A and E.—Therefore the rays which proceed from a light placedin the focus of a concave mirror fall divergent upon it,and are reflected parallel; it is exactly the reverse ofthe former figure, in which the rays fell parallel on themirror, and were reflected to a focus. In other words,when the incident rays are parallel, the reflected raysconverge to a focus; when the incident rays proceedfrom the focus, they are reflected parallel; this is a veryimportant law of ON REFRACTION AND COLOURS. Refraction is the effect which transparent mediumsproduce on light in its passage through them. Opaquebodies reflect the rays, and transparent bodies transmitthem; but it is found that if a ray, in passing from onemedium into another of different density, fall obliquely,it is turned out of its course. The power which causesthe deviation of the ray is not fully understood; butthe appearances are the same as if the ray (supposing ito be a succession of moving particles, which is for thipurpose the most convenient way of considering itwere attracted by the denser medium more strongly thanby the rarer. Let us suppose the two mediums to beair and water; when a ray of light passes from air intowater, it appears to be more strongly attracted by thelatter. If then a ray, ab, fall perpendicularly on water, REFRACTION AND COLOURS. 339 the attraction of the water acts in the same direction asthe course of the ray; it will not, therefore, cause adeviation, and the ray w
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